Organic petrologic and geochemical studies on the Oronto Group Nonesuch Formation (Middle Proterozoic) of the Midcontinent Rift System, northern Wisconsin and Upper Peninsula Michigan

Despite a recent revival in Precambrian organic geochemical studies, the relationship between organic primary production, preservation and diagenesis and sedimentologic processes prevailing in the Precambrian receives little attention. The present study seeks to, through the integration of diverse lines of geochemical data, suggest factors that account for the variability of geochemical parameters observed in Precambrian sedimentary sequences.; The organic-rich Nonesuch Formation comprises the middle portion of the Middle Proterozoic Oronto Group, part of a thick volcaniclastic rift fill sequence deposited in the Midcontinent Rift System (northern sector). As this formation is of disputed origin (i.e., marine versus lacustrine), hosts native copper mineralization and is prospective for hydrocarbons, an opportunity exists to examine topics important to workers interested both in economic and scientific aspects of the Precambrian.; Although stratigraphic/sedimentologic features of the Nonesuch Formation are similar in the east (Michigan) and west (Wisconsin) portions of the study area, distinctions in organic petrologic, elemental and stable isotopic parameters are noted. The TOC and Rock Eval HI values are lower in the west. Cross correlation of parameters related to the ancient Nonesuch Formation carbon cycle suggests indirect evidence for a relationship between primary productivity and carbonate precipitation in the east and direct evidence for organic degradation in the west. Such relationships are also indicated from specialized {dollar}deltasp{lcub}13{rcub}{dollar}C-pyrolytic/extract molecular studies and sulfur elemental and nitrogen stable isotopic data. It is proposed that whereas preservation of a particular primary producer may be nearly complete in one organic facies in both the east and west, organic remains in other western intervals were subject to extensive degradation and reconstitution.; The elemental, stable isotope and molecular parameters generated for the present study, in addition to detailing the causes of variable hydrocarbon prospectivity for east and west portions of the study area, afford the opportunity to observe Precambrian elemental and stable isotopic relationships (C, S, N), examine the utility of geochemical parameters in recognizing pre-Devonian lacustrine versus marine euxinic systems, assess the complexity of Middle Proterozoic aquatic ecosystems and recognize the influence of hydrothermal fluid contribution to ancient water body chemistry and early sediment pore waters.